This invention relates to a novel apparatus for depositing the air-borne cellulose or other fibers, including all forms of superabsorbent and wicking fibers, into foraminous forming pockets to form a fibrous pad. Prior art patents have demonstrated the importance of various methods of providing the fiber-air mixture to the foraminous forming pockets. U.S. Pat. No. 5,097,574 to Hertel et al. discloses the use of a reverse bend section in the fiber delivery duct which design intensifies the fiber stream to achieve particle deposition on the screen without substantial turbulence. U.S. Pat. No. 5,44,052 to Hertel et al. discloses the use of ducts that are characterized by reverse bends in order to densify the particle stream at the outside of the curvature for generally perpendicular deposition on the screen.
A major weakness in much of the art is the high pressure drops between the intake and outlet plenums as well as disturbances in air flow due to non-symmetrical placement of the exhaust from the low pressure plenum. A large body of art exists showing pad formers with the individual pockets having up to 8 layers of wire mesh, masks, and perforated plates which creates significant pressure drops across the forming section as well as highly turbulent areas above and below the forming section. Since the absorbent pads of today, especially in infant diapers are as much as 65% lighter than equivalent products of twenty years ago, due to the use of superabsorbents, uniform fiber formation and density are extremely important to the successful operation of pad formers.
It is well known in the patent art that in order to obtain good pad integrity it is necessary to have a high air flow through the pad during forming sequence. However, the large volume of air produces air turbulence and instability especially at the ends and edges of the forming pocket, consequently fiber distribution problems will occur. These instabilities are usually seen as eddies or pulsing at the perimeter of the foraminous forming pockets. This is especially bad when high differential pressures are created over the forming screen interface. This can be caused by excessively high inlet and outlet velocities, excessive under-screen masks and multiple layers of screens and other equipment.
Additional problems typically encountered due to uneven or turbulent flow entering the forming screen are uneven fiber density in the formed pad and clumping due to scarfing of the pad by high velocity air tangent to the screen.
After considerable investigation into the aerodynamics of the inlet and plenum we discovered that a significant slowing of the airflow from the conveying line into the forming area by a specified amount induced by a specific shaping of the inlet plenum resulted in improved uniformity of the pad. Additionally it was discovered that shaping the outlet or low pressure plenum in a specific way such that the airflow was gradually increased from the velocity at the screen surface to a specific range of velocities related to the area ratios of the inlet to outlet of the outlet plenum resulted in further improvement in pad density uniformity. It was also noted that the longer the effective length of the outlet plenum the more uniform was the fiber distribution.
This work was conducted using a rectangular pad designed for constant thickness from end to end and side to side to simplify the measurement of density variations. Limited work with pads having a shaped cross section indicated that end to end densities were uniform at given distances from the centerline of the pad.
In evaluations of competitive equipment before modification with low differential pressure plenums it was determined that the pressure profile across the width of the drum below the inner diameter of the drum had significant variations due to problems with the internal low pressure side design. This resulted in variations in fiber density and the amount or weight of fiber per square inch across the pad width and length. Use of the low differential pressure plenums resulted in reduced turbulence and improved pad density uniformity. It is important to note that the length of the outlet plenum provides a smooth transition back to high speed flow thus preventing the occurrence of eddies and turbulence on the inlet side of the forming screens. This results in a relatively even pressure distribution at all points on the forming screen. As mentioned above the experiments were carried out on a rectangular constant density absorbent product. However when the apparatus was used with shaped pads the uniformity was also excellent.
In another embodiment a superabsorbent doing tube is inserted through the intake plenum for delivery of superabsorbent. Tube placement is dependent on the anticipated placement of the superabsorbent in the pad.
These unexpected results are highly important in the modern absorbent pad where thinner pads are the rule. If the density and formation cannot be closely controlled the pad will break and not perform correctly.
It should be recognized that the instant concept is not limited to drum formers but can be adapted to other former configurations including linear endless "belt" systems.